1. Field of the Invention
Suspended cables, and particularly conductors for overhead electric lines, are known to vibrate due to the action of wind and such vibrations can lead to fatigue failure of said cables.
The frequency of said vibrations is substantially proportional to the wind speed.
For each type of electric cable or conductor, and for each value of the tensile stress to which said cable is subjected, it is possible to define the range of vibration frequencies which are dangerous for the integrity of said cable.
As known, the most adopted method for damping said vibrations is to use the so-called "vibration dampers", which are anchored in predetermined positions along the cable or conductor (see FIG. 1a).
2. Description of the Prior Art
The most widely used dampers are known as "Stockbridge" dampers, as described in the U.S. Pat. No. 1,992,538, granted on Feb. 26, 1935. They consist of a length of flexible steel cable, called "messenger cable", having the central part fixed to the conductor by means of a connection clamp, and of two counterweights fixed to either end of the messenger cable.
A damper constructed according to this known technique is shown in FIG. 1 of the accompanying drawings. It can be seen therein that the messenger cable 1 is a steel plait length and that the counterweights 4,4 have a bell shape. This shape allows to fix the counterweight at the end of the messenger cable, but at the same time it allows to position the barycenter (G) at a certain distance (a) from the end of said cable, and precisely in an intermediate position of the working length (l) of said cable.
The counterweights are normally obtained by iron casting or by casting of other alloys with specific high weight, or at times by pressed steel casting.
It has been possible to establish that dampers of this type have a marked damping effectiveness at their proper resonant frequencies, but far less effectiveness at different frequencies.
A suspended electric cable or conductor, provided with dampers of this type, may thus result well protected against the vibrations corresponding to said resonant frequencies, but insufficiently protected at the other frequencies.
A progress in this respect was obtained through the dampers according to the Italian Pat. No. 791,112 and No. 890,900 (or the corresponding U.S. Pat. No. 3,432,610 and No. 3,584,133) owned by the same Applicant, which have four resonant frequencies, instead of the only two frequencies found in the dampers of previous technique.
It is known that the damping characteristics and the resonant frequencies proper to a damper of the type heretofore described depend on different parameters, among which:
the counterweight mass, PA1 the moment of inertia of the counterweight in respect of its barycenter, PA1 the working length (l) of the cable, PA1 the distance (a) of the barycenter (G) of the counterweight from the end of the working length of the messenger cable. PA1 the first drawback is that, in order to obtain the desired global damping effect, it is necessary to use a larger number of dampers, with consequent higher costs; PA1 a second possible drawback is that, in the event of using dampers having an excessive dynamic reaction, the conductor risks damage at the point of anchorage of the connection clamp.
No absolute best values exist however for these parameters, which should instead actually be chosen in function of the vibrations to which a specific electric cable or conductor can be subjected, and of the required damping, particularly, in function of the range of dangerous frequencies and, for each frequency, of the damping effectiveness required to limit the vibration amplitudes to sufficiently low values so as not to produce a dangerous result.
Moreover, in the event that the damping of a given span of cable or conductor suspended between two extremes should require more than one damper, the above parameters should be chosen so as to distribute the resonant frequencies, proper to each of the dampers employed, in appropriate manner within the range of the dangerous frequencies.
The vibration conditions of the conductors are so varied and diversified that in order to apply, in each condition, dampers with optimized parameters, it would be necessary to employ a considerable number of counterweights of different shape and weight. Such an availability would, however, involve production costs (for the casting dies and relative depreciation) and storage costs which are so high as to be incompatible with present commercial requirements.
In practice, therefore, only about ten types of dampers are produced, or even less, and they are chosen almost only in function of the outer diameter of the cable or conductor.
Such a choice always forms a compromise, and it should in fact be noted that, very often, the dampers applied to the conductors have characteristics which differ from the optimal ones, whereby they provide the following drawbacks: